Semiconductor manufacturers have used various types of materials to make field effect devices, such as metal semiconductor field effect transistors (MESFETs), that are able to satisfy a wide variety of specific electronic design applications. Gallium Arsenide (GaAs) and Indium Phosphide (InP) semi-insulating materials have been widely used in the manufacture of lateral MESFETs. More recently, Silicon Carbide (SiC) and Gallium Nitride (GaN) have been investigated as promising materials for lateral MESFETs. While GaAs works well in most low voltage MESFET applications, its semi-insulating substrate has an inherent defect which limits the critical electric field strength corresponding to breakdown. For example, FETs made on semi-insulating GaAs tend to breakdown at drain to source voltages of less than 35 volts. When the gate potential in such a FET is applied to pinch off current flow from the source to the drain in the channel layer of the device and a high drain to source bias voltage is applied, fringing electric fields extend into the underlying GaAs semi-insulating substrate. That is, parasitic current flow occurs in the GaAs semi-insulating substrate at source to drain voltages in excess of about 35 volts.
The above-discussed material limitation makes GaAs unsuitable for high voltage applications in lateral MESFETs. Consequently, GaAs MESFETS cannot be used to generate high power at high voltage in conjunction with high impedance RF load applications, such as commercial microwave heaters, electrodeless lamps and base station cellular phone systems. While the current flow through a GaAs MESFET can be increased substantially for increased power by increasing the device periphery, impedance matching of such loads as mentioned above becomes difficult (inefficient impedance transformers are generally required). Extending the voltage capability of the device will allow higher RF voltage swings which will yield high power operation while maintaining impedance matching for commonly encountered loads without the use of transformers.
The inability of GaAs semiconductor material to isolate current flow to the channel region of the MESFET is a main reason why it has not yet been widely adopted for high voltage applications. Tens of millions of research dollars have been spent with respect to GaAs in an attempt to develop this material as a substrate for high voltage semiconductors, such as high voltage FETs. However, the industry has not been able to overcome the inherently low withstand voltage of GaAs "semi-insulating" substrates.
It would be desirable to use GaAs as a substrate for high power semiconductor devices. However, the inherent defects or impurities of GaAs that lead to low breakdown electric fields, as discussed above, present major obstacles for its widespread adoption in the high voltage semiconductor field.
It would also be desirable to use other materials, such as SiC, GaN and InP, in high voltage semiconductor applications. However, these materials are also unable to confine electron flow to the channel region of the device at high operating voltages.